Merge branch 'const_set_mod_to_s' of https://github.com/christopheraue/mruby into christopheraue-const_set_mod_to_s

It will no longer shrink arena region. Instead `vm.c` uses
a static function `mrb_gc_arena_shrink()` to shrink.

added tests for #3736

added basic tests for while/until/break/next

This change was suggested by Akira Kakuto.

fixed printing failed assertions

Should only check frozen fix #3737

This is a workaround for mingw-w64 (5.3.1) bug.

For readability's sake.

There's a number that negation does not work (-2147483648 in 32bit
environment).

Since there's no unsigned integer in Ruby. Binary, octal and
hexadecimal negative numbers can be represented by using 2's
compliment. But decimal (not being power of 2) cannot be use
that kind of format.